FIELD OF THE INVENTION
The invention relates to a method and a device for transferring a sheet, by a trailing edge thereof, in a reversing device of a sheet-fed rotary printing press, and more particularly, a sheet conveyed on an upline sheet-conveying cylinder, in particular, an impression cylinder, to a gripper device formed with sheet-retaining surfaces for receiving the trailing edge of the sheet, the gripper device being provided on a downline sheet-conveying cylinder in the reversing device of the sheet-fed rotary printing press.
In order to print on both sides of sheet-like material in a sheet-fed rotary printing press during a recto/verso or first-form and perfector printing operation, it is necessary to turn the sheet over with a reversing device, after the upper side of the sheet has been printed, in order to be able then to print on the rear side of the sheet.
The Published Non-Prosecuted Patent Application (DE-OS) 24 51 987 discloses a reversing device wherein, during recto/verso or first-form and perfector printing operation, the trailing edge of a sheet conveyed on an impression cylinder is gripped by a suction gripper pivotable outwardly from the periphery of a downline sheet-conveying cylinder, and before passing a central transfer location between the two cylinders, is guided into the periphery of the downline sheet-conveying cylinder. In the periphery of the downline sheet-conveying cylinder, the sheet trailing edge is then transferred to a further gripper device, the latter transferring the sheet trailing edge to a third gripper device, which is likewise pivotably arranged in the periphery. In order to counteract a relative movement between the sheet trailing edge and those parts of the sheet which still adhere to the impression cylinder, a movement which leads to errors, the suction gripper is guided on a trochoidal path when it is being moved into the periphery of the downline sheet-conveying cylinder. Because the transfer of the sheet trailing edge takes place in the interior of the periphery, and because of the complexity of the transmissions which are used, register-maintaining transfer is not possible with the aforedescribed reversing device, in particular, at high continuous printing speeds. In addition, in the hereinaforedescribed device, centrifugal force tends to produce a billowing in the sheets, particularly those having greater paper weights, more specifically, in a trailing part of the sheets which is lifted from the impression cylinder by the suction grippers, and this additionally presents an obstacle to the in-register takeover of the sheets by the succeeding gripper devices.
The published German Patent Document DE 38 29 626 C2 describes a sheet-fed rotary printing press having a reversing device wherein, during a perfector printing operation, the sheet trailing edge is lifted from the upline impression cylinder by a suction gripper that can be pivoted out of the periphery of a downline sheet-conveying cylinder. When the suction gripper is moved back under the periphery of the downline sheet-conveying cylinder, the movement of the suction gripper is controlled by a transmission so that the sheets are tautened counter to the direction of rotation of the downline sheet-conveying cylinder.
Due to the transfer of the sheet trailing edge by the suction grippers within the periphery of the downstream sheet-conveying cylinder in the devices according to the aforementioned published German Patent Document DE 38 29 626 C2 and German Non-prosecuted Patent Application (DE-OS) 24 51 987, comparatively large relative movements occur between the sheets and the gripper devices arranged in the interior and, in the case wherein there is no follow-up control for the grippers and suckers which are used, these relative movements may lead, for example, to so-called "picking", because the gripper holding the sheet leading edge remains closed, which, in turn, involves serious register problems. Furthermore, because of unequal closing and unequal opening of the grippers, respectively, as often occurs between the drive side and the operator side of a sheet-fed rotary printing machine because of the elasticity of the gripper shafts, it is possible for "picking " to occur at different times between the drive side and the operator side. The causes for the varying gripper closure are dynamically induced and are produced, for example, by varying gripper shaft torsion, differences in rotary speed, and so forth. Furthermore, in particular, in the case of thick printed materials, it is possible for stresses which cause register errors to occur in the sheet. These stresses can be attributed to the fact that the joint gripper closure period between the opening of the leading-edge gripper of the impression cylinder and the closing of the trailing-edge gripper, which is usually in the range of about 1 to 2 degrees of arc of the printing press, is further enlarged as a result of the increased thickness of the printed materials.